This invention pertains to the art of electroplating and more particularly to electroplating with high power requirements such as is necessary for the application of chromium.
It is well-known that depositing coatings of such materials as brass, nickle, chromium or zinc reduces corrosion and improves appearance of articles. The current practice includes placing the articles or workpieces to be plated into a rotating drum or work barrel which is in turn placed in an electrolyte comprising the appropriate plating substance. The articles are electrically connected to a cathode terminal of a power supply. Several varieties of cathodes have been implemented. A preferred variety is a dangler-type cathode conductor which is placed into the drum assembly. The corresponding anode of the power supply, being the opposite pole to that of the danglers, is applied to the electrolyte thereby completing an electrical circuit and causing migration of the plating ions to the objects to be plated. Significantly higher current densities are necessary with materials such as chromium. The work barrel is rotated to allow all articles therein to come into contact with the dangler assemblies, allowing for uniform plating of substantially all of them as well as keeping the electrolyte circulating into the plating barrel in a relatively homogeneous mixture.
In practice, the current density varies across the surface of the workpiece. However, effective plating with chromium occurs only within the range of current densities between 5 and 1000 amps/ft..sup.2. The ability of a plating solution to produce bright deposits over a range of current density is called its covering power, and the ability to deposit metal of even thickness at different current densities is called throwing power. Due to the positive slope of the current density-plating thickness curve, more metal is deposited on high current areas such as edges than is necessary for production. In certain areas of the workpieces, deposition of chromium in a non-decorative, roughened and tarnished form, known as burning, appears due to poor throwing power.
The apparatus of the present invention is particularly applicable to suitable high current chromium plating apparatus and will be described with particular reference thereto, although it will be appreciated that the invention has broader applications, such as the prevention of burning through more uniform power distribution to the workpieces.